The general field of this invention is the production of heavy oil and bitumen from underground deposits, particularly tar sands deposits. More specifically, this invention relates to a method for bitumen or heavy oil recovery by in situ combustion that substantially decreases carbon dioxide emissions.
There are extensive oil sand reserves in many areas of the world, e.g. Alberta in Canada, Utah, Wyoming, and Colorado in the United States, and Venezuela. However, the very high viscosity of bitumen makes economically viable and environmentally responsible exploitation of these abundant resources very challenging. Conventional oils can be efficiently recovered without any need for heating to reduce their viscosity. Carbon dioxide injection can be used to reduce the viscosity of conventional oil and increase the quantity of oil recovered. However, substantial energy inputs are required to lower the viscosity of very heavy oil or bitumen in order to achieve reasonable production rates. This high energy input requirement increases the bitumen production cost and associated carbon dioxide emissions relative to conventional oil. As a result of the dwindling reserves of conventional oil, there has been a long-standing effort to develop methods to more economically exploit these reserves in an environmentally responsible manner.
Bitumen was initially produced by mining using the ‘hot water’ method (CA2004352) to extract the hydrocarbons from the mineral gangue. This method has four problems. First, mining requires disturbance of the land. Second, most bitumen reserves are too deep to be economically recovered by mining. Third, this method requires large quantities of scarce water. Fourth, this method has a high energy input requirement, which substantially decreases the net energy production and increases CO2 emissions relative to conventional oil production.
The steam assisted and gravity drainage (SAGD) in situ bitumen production method (U.S. Pat. No. 4,344,485) is currently the most commercially successful method to exploit deeper bitumen reserves. This method uses a horizontal upper steam injection well and lower parallel bitumen production well. Although this method can economically produce bitumen at greater depths than mining, it also requires large quantities of water and energy for steam production. Boilers (US 2007/0266962) with indirect heating are typically used for SAGD steam production, which generally require a premium natural gas fuel and results in significant CO2 emissions. U.S. Pat. No. 4,224,991 teaches an oxygen-fuel combustion method with water injected into the combustion region to control its temperature to produce steam for heavy oil production. In addition, direct contact steam generators can also advantageously use water to control flammability of the premixed O2-fuel feed (U.S. Pat. No. 6,206,684) or to cool the combustion chamber walls (U.S. Pat. No. 2,359,108).
It has long been recognized that in situ combustion bitumen production processes have the potential to solve many of the SAGD process problems by [1] producing, rather than consuming water, [2] using lower value bitumen components as an energy source, rather than an external premium fuel, [3] in situ upgrading of the bitumen, and [4] simplifying the recovery of CO2. In situ combustion processes typically have vertical oxidant injection wells and vertical (U.S. Pat. No. 4,722,395) or horizontal (U.S. Pat. No. 5,456,315) production wells. A wide variety of oxidants have been proposed for in situ production processes: air (U.S. Pat. No. 5,456,315), air-O2 (U.S. Pat. No. 4,557,329), O2-water (U.S. Pat. No. 5,027,896), O2-steam (U.S. Pat. No. 4,133,382), O2—CO2 (U.S. Pat. No. 4,410,042), and O2—CO2— steam (U.S. Pat. No. 4,217,956). Hydrotreating catalyst may be advantageously added to the production well for in situ bitumen upgrading using in situ produced hydrogen (U.S. Pat. No. 6,412,557). There have been modest efforts to utilize the off-gas from in situ combustion bitumen production processes. For example, U.S. Pat. No. 4,454,916 teaches the use of a highly enriched air oxidant (≧50 volume percent O2) separating carbon dioxide from the off-gas from an in situ heavy oil production process to produce a low heating value fuel gas.
A number of factors make carbon dioxide capture and sequestration from conventional bitumen production processes impractical. First, the gaseous emissions from bitumen production processes invariably have very low carbon dioxide concentrations, typically about 15 molar percent. Second, bitumen production facilities typically have a large number of gaseous emission points. Third, bitumen production facilities have a much shorter economic life than most other large carbon dioxide emission sources like coal fired electrical power generation facilities. Unfortunately, bitumen production processes have high carbon dioxide emission rates. For example, mining and SAGD bitumen production processes have typical carbon dioxide emission rates of about 90 and 60 kilograms per petroleum barrel (kg/bbl) bitumen produced. As a result, carbon dioxide emissions from bitumen production will likely become a significant source of carbon dioxide emissions and have a significant impact on global warming. For example, bitumen production from Canadian oil sands is expected to increase from 1.3 million barrels per day in 2008 to about 4 million barrels per day by 2030. If conventional technology were used to meet this expected increase in market demand, then the associated carbon dioxide emissions should increase by about 60 million metric tons per year by 2030, or about 10% of Canada's 2006 total carbon dioxide emissions. Therefore, there is a clear need for a practical bitumen production method with much lower carbon dioxide emissions that is not met by the prior art.